OPN is a secreted acidic phosphorylated glycoprotein with a molecular weight of about 41 kDa identified as a major non-collagenous protein constituting the matrix of bone tissue where calcium is deposited. OPN is widely expressed in milk, urine, renal tubular, osteoclasts, osteoblasts, macrophages, activated T cells, and various tumor tissues. OPN has been considered to play a role in anchoring osteoclasts to hydroxyapatite in bone matrix (Nonpatent Literature 1), but other various functions of OPN have been reported such as involvement in cell adhesion, cell migration, control of nitric monoxide production, tumors, and the immune system.
The expression of OPN correlates with tumor progression and has an association with cancer metastasis. OPN has been detected in plasma of patients with lung cancer, liver cancer, breast cancer, or prostate cancer (Nonpatent Literature 2). It has been reported that the expression of OPN mRNA in a cancer site is higher than that in a normal site (Nonpatent Literature 3), and it has also been reported that the expression of OPN in glioma tends to correlate with the degree of malignancy (Nonpatent Literature 4). The correlation between OPN expression and tumor has been confirmed also in animal models (Nonpatent Literature 5). Based on these recent findings about OPN, suppression of production of OPN that promotes metastasis and invasion of tumor cells has come to be considered as one of new approaches of an anti-cancer drug that prevents cancer metastasis (Nonpatent Literature 6).